Screeds for use with paving machines for compacting and smoothing asphalt paving composition are well known. Typically, the screed is mounted behind the paving machine on the end of a towing arm, and the forward end of the towing arm can be raised and lowered to vary the angle of attack of the screed base plate with respect to the surface being paved. The paving machine lays down a mat of asphalt mix ahead of the screed, and the screed smooths and precompacts the mat. An automatic grade control regulates the thickness of the material placed into the road mat by raising and lowering the forward edge of the towing arm to vary the angle of attack of the screed base plate. As the tow point is lifted up, the leading edge of the screed is lifted up, causing more mix to go into the mat. As the tow point drops down, the leading edge of the screed drops down, causing less mix to go into the mat.
Where the desired width of the mat being laid is wider than the standard screed, a variable width screed is used. With this arrangement, a front or main screed is mounted to the rear end of the towing arm, and a rear of variable screed is attached to the rear of the main screed. The rear screed is laterally extensible to either side of the main screed to smooth and compact the mat beyond the outer edges of the main screed.
A number of fundamental problems occur when the variable width screed is added to the main screed. When the tow point is dropped down as required by the automatic grade control to reduce the thickness of the mat, the variable screed attached to the back of the main screed is forced to lift up, since the screeds rotate around the trailing edge of the main screed. Thus, while the main screed is being lowered to put less mix in the mat, the variable screed is being raised, which has the effect of placing more mix in the mat. Conversely, as the automatic grade control lifts the tow point, the screeds tend to rotate around the trailing edge of the rear screed, causing the rear screed to dig into the mat and lifting the front screed completely off the mat. Accordingly, any automatic adjustment by the grade control of the tow arm to adjust the angle of attack of the main screed causes the opposite reaction to occur with the variable screed.
Further, when width changes are made by extending the variable screeds outwardly, the weight of the two screeds remains constant, but the area of mix supporting the screeds increases. Therefore, the screeds can compact the mix less, causing the rear of the main screed to lift up and, therefore, the towing arm to drop down to compensate for it. This, in turn, causes the rear screed to raise up, again causing irregularities in the mat.
Accordingly, there is a need to provide a variable width screed which avoids irregularities in the mat resulting from the rear screed reacting oppositely to the front screed as the screeds are adjusted to vary the thickness of material going into the mat.
A further problem arises when screeds are stopped and restarted during the paving process. When the screeds are paving, they are running at a given angle of attack. As the paver stops at the end of one truckload of mix, and while the next truck is backing into position to deliver additional mix, the paver may be idle for some period of time, permitting the mix under the screed to become cold. Upon restarting, the screed can jump and slide on the ramp created by the angle of attack of the screeds against the mat. This causes too much mix to be placed in the mat directly ahead of the point where the screed was stopped.
Accordingly, there is a need to provide an automatic device that will avoid the uneven placement of mix in the mat when the paving machine is stopped and restarted.